Fire is a gas. The only reason it doesn’t “look” like a gas to the human eye is cause only part of it glows and as it cools it becomes invisible. Fire expands outwards and upwards as it cools and dissipates into the atmosphere, more or less the same way any other hot gas would.
In fact you could say that fire is specifically the hot and visibly incandescent portion of a gas cloud formed by the chemical reaction of fuel and heat and oxygen.
Note: really really hot fires can generate plasma, like thermite for example, but your average fire made with wood or gas or other common household fuels would generate little to no plasma.
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